1. Field of the Invention
This invention relates to telephone circuits.
2. Description of the Prior Art
In known telephone circuitry which uses electronic diallers, bipolar switches incorporated in the diallers have a high saturation resistance when driven by low base bias currents. This has resulted in an increase of up to 50 ohms in set resistance on 20 mA lines. Since linear devices have been required in the circuitry, problems with high AC loading of the voice network due to the circuits required to drive the dial switch have been encountered and these are difficult to surmount. Also, distortion problems occur in both the transmit and sidetone characteristics of voice networks. To overcome this, extra circuit elements are required in optimum bipolar line pulsing circuits and these elements require relatively large input control currents, i.e. in the range of 10% of the main or line current.
In addition, transitionized switching is usually used for dialling and is arranged so that the dialling signal bypasses conventional voice networks. This is because conventional voice networks are known to distort dialling wave forms and thus effectively alter the dialling signal.